This invention relates to methods and apparatus for noninvasively, continuously and discretely monitoring dynamic cardiac performance of a living subject.
Cardiac output is an important parameter of the entire circulatory system since it determines the blood flow, and thereby, the transport of oxygen and nutrients, to all tissues of the body. Heart disease can result in a decrease of cardiac output leading to inadequate nutrition of the cells of the body. Therefore, measuring cardiac output is useful in monitoring the critical ill patient, in rehabilitation medicine, and in medical screening.
Cardiac output may be expressed as the product of heart rate and volume of blood pumped per beat of the heart. Thus, under conditions of a consistent heart rate and stroke volume; ##EQU1##
Cardiac output or blood flow is also directly proportional to mean blood pressure and inversely proportional to peripheral resistance of the artery through which the blood flows (i.e. the aorta).
Because of the importance of changes in cardiac output and the difficulties in its direct measurement, the estimation of cardiac output and stroke volume from blood pressure pulse waveforms has been extensively studied. McDonald (1974) recorded two pressure pulses 3-5 cm apart in the ascending aorta. Both pulses were subjected to Fourier analysis and the apparent phase velocity was calculated for each harmonic of the pulses. The phase velocities were applied to the Wormsley equation to calculate aortic flow and stroke volume (the integral of aortic flow throughout one cycle). A problem encountered with this method is that the aorta exhibits non-uniform geometric elastic properties. To overcome the problem, Muthukrishnan and Jaron (1975) used a parameter optimization technique to compute aortic input impedance in a manner similar to Strano, Welkowitz and Fich (1972) based upon an aorta model developed by Welkowitz and Fich (1967). Instantaneous aorta flow waveforms were then calculated from the input impedance and the proximal aortic pressure. The aortic flow waveform estimated by this analysis closely matched the waveform measured using an electromagnetic flow meter. This method, however, which is based upon two pressure measurements, requires an invasive stroke volume estimation.
It is known that the hemodynamic characteristics of the aorta can be simulated by an R-L-C linear electrical network. Researchers have developed various aorta simulation models to perform estimations and calculations for different purposes. Based upon an equivalent electrical circuit model developed by Watts (1974) the aortic flow waveform can be calculated from the carotid input pulse waveform. A corresponding cardiac output then may also be computed. A microcomputer can be used for this simulation and calculation. The knowledge that the aorta can be represented by an electrical circuit model has led some researchers to seek a non-invasive method and apparatus for cardiac output monitoring using this circuit simulation.